Conventionally, in each of magnetic disks adapted to be mounted in magnetic disk apparatuses, such as hard disk drives (hereinafter abbreviated as HDDs), a protective layer and a lubricating layer are provided on a magnetic recording layer formed over a substrate, for a purpose of ensuring durability and reliability of the magnetic disk. Particularly, the lubricating layer used as the outermost surface is required to have various properties such as long-term stability, chemical substance resistance, friction properties, and heat resistance.
To satisfy such a requirement, perfluoropolyether-based lubricants having hydroxyl groups in molecules have often been used conventionally as lubricants for magnetic disks. For example, JP-A-S62-66417 (Patent Document 1) or the like discloses a known magnetic recording medium or the like coated with a perfluoroalkylpolyether lubricant having a structure of HOCH2CF2O(C2F4O)p(CF2O)qCH2OH. In this lubricant it is known that when hydroxyl groups are present in molecules of a lubricant, adhesion properties of the lubricant to a protective layer can be obtained by the interaction between the protective layer and the hydroxyl groups. As commercial products of perfluoropolyether-based lubricants, use is often made of Fomblin Z-based lubricants of Solvay Solexis, Inc. having heat resistance and stability. For the purpose of removing impurities or the like from these commercial lubricants or for the purpose of obtaining a proper molecular weight distribution because of each of these lubricants being a polymer substance, various purifications have been carried out and purified products have been used as lubricants for magnetic disks.
Further, JP-A-2006-70173 (Patent Document 2) discloses a lubricant containing polyether fluoride having a structural unit with hydroxy groups and CF2 at both ends, having a number-average molecular weight of 4000 to 12000, and having in average 1 to 10 hydroxy groups on the midway of the molecular chain per molecule.    Patent Document 1: JP-A-S62-66417    Patent Document 2: JP-A-2006-70173